This invention relates to the field of additive metallization of substrates. Many electronic applications require patterned metallization of nonconductive substrates for interconnection among electronic devices. Examples of such applications include high density packaging (multi-chip modules), antennas, flex circuits, printed wiring boards, and flat panel displays. The metallic interconnects are conventionally formed by subtractive processes. Modern additive processes attempt to overcome the drawbacks of subtractive processes.
In a subtractive metallization process, a surface of the substrate is first fully coated. Alternatively, a metal sheet can be laminated onto a flat substrate using adhesive. Selected portions of the metal plating are then etched to leave the desired patterned metallization. Vacuum-assisted physical vapor deposition and sputtering deposition are often used to achieve en masse plating. Physical vapor and sputtering deposition require high vacuum, and consequentially high capital equipment and operating costs. Those deposition processes can also result in poor adhesion of the metal to the substrate.
Full coverage metal plating can also be achieved by sensitizing a surface of the substrate with a palladium chloride/tin chloride bath and chemically reducing palladium ions to form catalytic clusters. Electroless plating followed by electrolytic plating deposits metal on the surface. This process can be costly due to the large number of wet processing steps, and the chemical dissimilarity between the metal coating and the substrate discourages chemical bonding therebetween. Consequently, the metal only weakly adheres to the substrate.
After full coverage metal plating, a layer of resist (a photoresist is often used) is deposited in a pattern on top of the metal layer, with the pattern corresponding to the desired metallization pattern. A subsequent etching step removes all the metal except that protected by the patterned resist layer. The etching process is usually time consuming and costly, and can require the use of materials unfriendly to the environment.
Additive processes have been proposed to overcome the environmental drawbacks of subtractive processes. Current additive processes achieve only thin layers of metallization, limiting their practical applications. See, e.g., Tokas et al., U.S. Pat. No. 5,348,574. Current additive processes also suffer from poor adhesion, just as with the subtractive processes. Some of the additive processes retain resist and etching steps; consequently, they suffer from the same environmental hazards as the subtractive processes. See, e.g., Hirsch et al., U.S. Pat. No. 5,192,581. Others have process limitations that limit their uses with widely available substrate materials. See, e.g., Orlowski et al., U.S. Pat. No. 5,153,023. Metal/foil adhesive processes use costly thick metal foils and poorly adhering low molecular weight polymer adhesives.
Accordingly, there remains a need for improved additive metallization processes, specifically for processes that provide fine line metallization and that can provide the increased metallization thickness required in practical applications such as printed wiring boards.